♻️ refactor(model): replace gorm.io/datatypes with JSONValue for PrefillGroup.Items; fix JSON scan across drivers

- Why:
  - Avoid introducing `gorm.io/datatypes` for a single field.
  - Align with existing pattern (`ChannelInfo`, `Properties`) using `Scanner`/`Valuer`.
  - Fix runtime error when drivers return JSON as string.

- What:
  - Introduced `JSONValue` (based on `json.RawMessage`) implementing `sql.Scanner` and `driver.Valuer`, with `MarshalJSON`/`UnmarshalJSON` to preserve raw JSON in API.
  - Updated `PrefillGroup.Items` to use `JSONValue` with `gorm:"type:json"`.
  - Localized comments in `model/prefill_group.go` to Chinese.

- Impact:
  - Resolves “unsupported Scan, storing driver.Value type string into type *json.RawMessage”.
  - Works with MySQL/Postgres/SQLite whether JSON is returned as `[]byte` or `string`.
  - API and DB schema remain unchanged; no `go.mod` changes; lints pass.

Files changed:
- model/prefill_group.go

web/src/components/common/ui/JSONEditor.js

@@ -637,7 +637,7 @@ const JSONEditor = ({
         {/* 额外文本显示在卡片底部 */}
         {extraText && (
           <Divider margin='12px' align='center'>
-            {extraText}
+            <Text type="tertiary" size="small">{extraText}</Text>
           </Divider>
         )}
         {extraFooter && (